🔹 Types of Point Defects

1. Vacancy Defect

• An atom is missing from its regular position in the crystal lattice.

• Common in metals and can increase with temperature.

• Causes a local distortion of the crystal.

🧠 Effect: Increases diffusion, may weaken mechanical strength.

2. Interstitial Defect

• An extra atom occupies a space between regular lattice points.

• Can be:

  • Self-interstitial: An atom of the same material.

  • Foreign interstitial: An atom of a different material (smaller in size).

🧠 Effect: Creates large local distortions, affecting strength and conductivity.

3. Substitutional Defect

• A foreign atom replaces a host atom in the lattice.

• Happens in alloying (e.g., copper-zinc in brass).

🧠 Effect: Alters electrical, thermal, and mechanical properties.

4. Frenkel Defect (mainly in ionic crystals)

• An ion is displaced from its lattice site to an interstitial site, creating both a vacancy and an interstitial.

🧠 Effect: Keeps charge balanced but affects ionic conductivity.

Example: Seen in AgCl, ZnS.

5. Schottky Defect (also in ionic crystals)

• A paired vacancy of one cation and one anion to maintain charge neutrality.

🧠 Effect: Reduces density, increases ionic conductivity.

Example: Found in NaCl, KCl, CsCl.